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The ASPCA is investigating what caused the carriage horse to fall. (Published Monday, Dec. 5, 2011)

The ASPCA said Monday it is investigating why a horse fell as it was leading a carriage carrying four people in Midtown.

The white carriage horse was photographed on the ground near the busy Grand Army Plaza area Sunday evening.

Animal activist groups happened to be protesting nearby when the horse went down.

One of the protesters, Mary Xanthos, a veterinarian with the group Win Animal Rights, told NBC New York, "A bunch of people were surrounding the horse and trying to take off some of the gear and help the horse up."

She said a driver told her the horse "just tripped."

The horse was able to get up, and the driver ostensibly took it back to the stables, said Xanthos.

It was not immediately clear who owned and operated the horse, which has not yet been identified.

The horse will not work until it is checked out by a veterinarian, the ASPCA said.

In late October, a white carriage horse named Charlie collapsed and died as it trotted toward Central Park. A necropsy determined Charlie was unhealthy and not fit to work as a carriage horse.